Factual error: The castle is built amongst beautiful scenery, however, there are no mountains like that in the Czech Republic.
Factual error: There are no mountains high enough near Prague. The closest mountains that fit the description are the Alps between Germany and Italy.
Answer: Vin Diesel chose to reject the sequel as he was being offered three different franchises, based on XXX, The Fast and the Furious concept and the character of Riddick (from Pitch Black) - he presumably felt that he couldn't commit to them all. In the end, he chose to go with the Riddick films and reject the others. The sequel is called XXX for the simple reason that the character is still present - they've just recast the role.
Tailkinker ★